1. Field of the Invention
The present invention generally relates to electric propulsion systems for marine vehicles, and more particularly to propulsor units used in connection with such electric propulsion systems.
2. Description of the Prior Art
Electric propulsion systems for use with both surface and submersible marine vehicles are well known in the art. For example, U.S. Pat. Nos. 4,831,297; 4,927,329; 5,078,628; 5,220,231; 5,252,875; and 5,289,068, all teach various types of electric propulsion systems adapted for use on such marine vehicles, which patents are hereby incorporated herein by reference.
These and other prior art electric propulsion systems typically consist of an energy source, an energy conversion system, and a propulsor unit. The energy source may be a nuclear power plant, an internal combustion engine, or even a battery. In the case where a nuclear power plant or internal combustion engine is employed, the output energy must be converted to electrical energy. Typically, this conversion is accomplished by mechanically coupling the output of the energy source to an ac or dc generator so as to produce an electric current that is adapted to power the propulsor unit.
In order to achieve a high propulsive efficiency--within specified noise and vibration criteria--a highly loaded, diameter-constrained propulsor unit is often employed. These prior art propulsor units typically comprise a conventional electric motor that is adapted to drive a propeller system. As a result of the noise and vibration criteria, these prior art propulsor units are designed to operate at relatively low rotational speeds. As a consequence, the propulsor unit must provide a very high output torque in order to achieve a high propulsive efficiency. Unfortunately, many of the prior art electric motor and propeller systems fail to provide the necessary level of output torque.
Often, the diameter of the propulsor unit may be constrained as a result of the severe dimensional restrictions dictated by the marine vehicle itself. In particular, the diameter of the propulsor unit is often limited by the hull dimensions of the vessel. The restricted diameter of the propulsor unit also tends to reduce the propulsive efficiency in prior art electric propulsion systems. In particular, the efficiency of prior art electric motor propulsor units often drops off dramatically at speeds lower than the "design" speed.
A further problem associated with prior art propulsion systems is the production of waste heat. In many cases, a substantial cooling system may be required to remove the excess heat that is produced by the energy source, the energy conversion system, and the propulsor unit. A typical cooling system may include a circulation pump, a heat exchanger, and associated piping. Such cooling systems not only occupy already limited space within the vessel, but also require energy to be diverted from the propulsion system to run the circulation pump. This problem is particularly troublesome when it arises in high performance motors of the type typically used in connection with driving torpedoes and the like. Such motors are often designed for optimum performance over a relatively brief period of time, e.g., 3-5 minutes. The additional weight and complexity associated with the inclusion of a typical cooling system in this type of marine vehicle is a significant disadvantage in the prior art.
Consequently, a need exists for an integrated motor/marine propulsor system which will eliminate the above-described problems associated with conventional electric propulsion systems while, at the same time, providing higher efficiency, power density, and output torque.